I recently heard from a long-time listener who was upset by what she felt was highly misleading nutrition information on one of her favorite products.

“Alice” explained that she’d been eating a lot of tortillas, in the form of breakfast tacos, wraps for lunch, and tortilla and peanut butter roll-ups for snacks. She had found a few different low-calorie tortillas, with anywhere from 30 to 50 cal according to their labels.

“But when I calculated the calories myself, by multiplying the grams of carbohydrate by 4, grams of protein by 4, and grams of fat by 9,” she wrote, “the result was almost double what the nutrition labels showed. If you are someone that only has a wrap on occasion, it's not a big deal. But if you are someone like me who eats several wraps per day, you might be eating a lot more calories than you think you are. How can they get away with this?”

In fact, the nutrition counts provided on these labels were perfectly legal, as well as reasonably accurate. There are a few reasons why the calorie counts on Nutrition Facts labels may not add up the way you think they should. One has to do with common misunderstandings about how many calories the so-called macros (carbs, fats, and protein) provide.

The 4-4-9 myth

It’s commonly believed that 1 gram of carbohydrate contains 4 calories, 1 gram of protein has 4 calories, and 1 gram of fat has 9 calories. These numbers are called Atwater equivalents, and they represent the average values for these macronutrients. In fact, a gram of carbohydrate may have anywhere from 0 to 4.5 calories, depending on the source of carbohydrate. There are also variations in the number of calories per gram in fats and proteins from different sources, although the range is quite a bit smaller.

Manufacturers have a number of options for calculating their calorie counts. The simplest way is to use the Atwater equivalents. For a lot of foods, this is going to be pretty close. But for some foods, using Atwater equivalents would significantly overstate the calories. This is especially true for foods that have been engineered to be lower in carbohydrates than they normally would, such as the low-carb tortillas that Alice was buying.

The total carbohydrate count that you see on the Nutrition Facts label includes starches and sugars, as well as fiber and sugar alcohols. But because these last two categories are largely indigestible, they contribute a lot fewer calories per gram than other carbohydrates. When you see a package that lists “net carbs” in addition to total carbohydrates, that’s often a food with a lot of indigestible carbohydrates.

Understanding net carbs

Net carbs is not an officially recognized way of stating the nutrient content of foods—and this number cannot be included in the Nutrition Facts label. It’s...

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